Need to use find and replace to change 1 digit in a 6 digit string of numbers

Modnar1 

Here's a single formula that will simply increment the first digit by one. So 1nnnnn becomes 2nnnnn, 2nnnnn becomes 3nnnnn and so forth. This uses the new LET function to make it more readable. You will need the newest version of Excel for this to work. It also assumes--which I think is the case--that this whole string is one. 

=LET(

idx,VALUE(LEFT(C3,1)),

NewLast,RIGHT(LEFT(C3,6),5),

NewFst,CHOOSE(idx,"2","3","4","5","6","7","8","9","1"),

NewFst&NewLast

)

It can be modified if those are not the replacements you desire (see below how NewFst works)

idx is an internal (to the formula) defined variable, containing the value of the first character

NewLast contains the last five digits of the first six digits

NewFst uses the CHOOSE function to pick, based on the value of idx, the text "2" or "3" etc. in sequence

The final formula concatenates the two text strings NewFst and NewLast to give your new six digit number.

I've attached a spreadsheet using this formula with some of the data from you, with the assumption, as noted above, that your data comes through as one solid text string.